The present invention relates to displays, and more particularly, to dynamically changeable displays visible as a result of reflected incident light.
Small displays are incorporated in many hand-held devices and the like recently. The most familiar examples are those employed in wristwatches and hand-held calculators. Since most such small devices are battery operated, battery life is a critical factor in the selection of components. Small displays using light emitting diodes produce their own light and are, therefore, viewable in the absence of incident light. On the negative side, however, light emitting diodes consume more energy than other available display types such as liquid crystals. Liquid crystals modify the light passing therethrough. For viewing in the absence of incident light, they must be lighted from a separate source. Since most viewing takes place in the presence of available incident light, a liquid crystal display can be configured in the manner of FIG. 1 wherein the liquid crystal display 10 has a mirror 12 containing reflective surface 14 attached on the back side. The liquid crystal display 10 is connected to be driven by the control circuit 16. Incident light 18 passes through the liquid crystal display, reflects off of the reflective surface 14 of mirror 12 and passes back through the liquid crystal display 10 to be viewed by the viewer whereby the display information impressed on the liquid crystal display 10 by the control circuit 16 can be viewed.
Recently, magneto-optic displays of the type described in my co-pending applications Ser. No. 375,322, titled SWITCHABLE TANDEM MEMORY MAGNETO-OPTIC DISPLAY and Ser. No. 375,325, titled MAGNETO-OPTIC DISPLAY both filed on even date herewith and assigned to the assignee of this application, have gained rapid popularity. The chip is divided into a rectangular pattern of posts with appropriate control wires disposed therebetween. Each individual post is addressible and can be magnetized in either of two directions. Depending upon the direction of magnetization, polarized light passing therethrough is rotated clockwise or counter-clockwise a fixed amount by the Faraday effect. By viewing the display through a polarized sheet having its polarization axis properly angled, the rotated light is placed closer to or further from axial alignment with the polarization axis of the analyzer sheet to, thereby, appear lighter or darker, respectively. Because of their mode of operation, such displays have been thought to be unusual in the manner of liquid crystal display described in FIG. 1. If, as shown in FIG. 2, a magneto-optic chip 20 connected to a driver circuit 22 with a polarizing sheet 24 placed over its front surface and a mirror 12 having a reflective surface 14 placed adjacent its back surface has incident light 18 reflected back through it in the manner of the liquid crystal display 10, the display information on the magneto-optic chip will be invisible. The reason for this can be seen with reference to FIGS. 3 and 4. Assuming that the polarizer 24 sets the polarization axis at 0.degree. and the magneto-optic chip 20 is imparting a 10.degree. rotation in either direction, after passing through the chip 20, the rotated portions of the potential display will be offset at +10.degree. and -10.degree., respectively. After reflecting off the mirror reflective surface 14, no rotation will be imparted so that the axial orientation will still be at +10.degree. and -10.degree., respectively. In passing back through the magneto-optic chip 20, a further 10.degree. of rotation will be effected in the same direction, as is characteristic of such chips. Thus, upon being viewed through the polarizer 24, the light will be rotated to an axial orientation of +20.degree. and -20.degree., respectively, with respect to the polarizer sheet's 0.degree. axial orientation; that is, both the background areas and the message areas will be offset equally in opposite directions from the vertical. The contrast between the background and message required for the message to be viewed is a function of the cosine of the angle. The cosine of +20.degree. and the cosine of -20.degree. are both 0.94. Consequently, both the background and message have identical amounts of light attenuation and, therefore, present a uniformly illuminated surrface to the viewer.
Wherefore, it is the object of the present invention to provide a magneto-optic display which is viewable as a result of reflected incident light only.